1. Field of the Invention
The present invention relates to a magneto-resistive tunnel junction head for reading the magnetic field intensity from a magnetic recording medium or the like as a signal and, in particular, to a magneto-resistive tunnel junction head which has a new design of biasing means for improving an output for adaptation to ultra-high density recording and is excellent in flexibility of selection of the biasing means.
2. Description of the Prior Art
MR sensors based on the anisotropic magneto-resistance (AMR) or spin-valve (SV) effect are widely known and extensively used as read transducers in magnetic recording. MR sensors can probe the magnetic stray field coming out from transitions recorded on a recording medium by the resistance changes of a reading portion formed of magnetic materials. AMR sensors have quite a low resistance change ratio xcex94R/R, typically from 1 to 3%, whereas the SV sensors have a xcex94R/R ranging from 2 to 7% for the same magnetic field excursion. The SV magnetic read heads showing such high sensitivity are progressively supplanting the AMR read heads to achieve very high recording density, namely over several Giga bits per square inch (Gbits/in2).
Recently, a new MR sensor has attracted attention for its application potential in ultra-high density recording. Magneto-resistive tunnel junctions (MRTJ, or synonymously referred to as TMR) are -reported to have shown a resistance change ratio xcex94R/R over 12%. Although it has been expected that TMR sensors replace SV sensors in the near future as the demand for ultra-high density is ever growing, an application to the field of the magnetic heads has just started, and one of the outstanding objects is to develop a new head structure which can maximize the TMR properties. Great efforts of developments are still needed to design a new head structure since TMR sensors operate in CPP (Current Perpendicular to the Plane) geometry, which means that TMR sensors requires the current to flow in a thickness direction of a laminate film.
In a basic SV sensor which has been developed for practical applications, two ferromagnetic layers are separated by a non-magnetic layer, as described in U.S. Pat. No. 5,159,513. An exchange layer (FeMn) is further provided so as to be adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent ferromagnetic layer are exchange-coupled so that the magnetization of the ferromagnetic layer is strongly pinned (fixed) in one direction. The other ferromagnetic layer has its magnetization which is free to rotate in response to a small external magnetic field. When the magnetization""s of the ferromagnetic layers are changed from a parallel to an antiparallel configuration, the sensor resistance increases and a xcex94R/R in the range of 2 to 7% is observed.
In comparison between the SV sensor and the TMR sensor, the structure of the TMR is similar to the SV sensor except that the non-magnetic layer separating the two ferromagnetic layers is replaced by a tunnel barrier layer being an insulating layer and that the sense current flows perpendicular to the surfaces of the ferromagnetic layers. In the TMR sensor, the sense current flowing through the tunnel barrier layer is strongly dependent upon a spin-polarization state of the two ferromagnetic layers. When the magnetization""s of the two ferromagnetic layers are antiparallel to each other, the probability of the tunnel current is lowered, so that a high junction resistance is obtained. On the contrary, when the magnetization""s of the two ferromagnetic layers are parallel to each other, the probability of the tunnel current is heightened and thus a low junction resistance is obtained.
U.S. Pat. No. 5,729,410 discloses an example wherein a TMR sensor (element) is applied to a magnetic head structure. The TMR sensor is sandwiched between two parallel electrical leads (electrodes), that are in turn sandwiched between first and second insulating gap layers to form a read gap. A pair of permanent magnets are formed to secure a single magnetic domain structure of a free layer so as to suppress generation of the Barkhausen noises. In this case, attention is paid to avoiding a contact between the pair of permanent magnets and the TMR sensor portion so as to prevent an electrical short circuit of an insulating barrier.
However, the TMR head structure proposed in U.S. Pat. No. 5,729,410 has problems that since the permanent magnet and the free layer are formed with a given distance therebetween, the bias effect is reduced, and that the biasing means are limited to permanent magnets due to magnetic separation between the biasing means and the free layer.
For solving the foregoing problems, the present inventors have attempted to design one head structure which is shown in FIG. 6 in section. The TMR head 100 shown in FIG. 6 is provided with a TMR element 200 in the form of a laminate body comprising a ferromagnetic free layer 120, a tunnel barrier layer 130, a ferromagnetic pinned layer 140 and an antiferromagnetic pinning layer 150, and further provided with insulating layers 191 and 191 formed at opposite ends (left and-right sides in FIG. 6) of the element 200. Magnetization of the ferromagnetic pinned layer 140 is fixed in one direction (depth direction of the drawing sheet) by the antiferromagnetic pinning layer 150, while magnetization of the ferromagnetic free layer 120 can be rotated freely in response to an external signal magnetic field.
Further, on upper surfaces at opposite ends of the ferromagnetic free layer 120 located at the top of the TMR element 200, bias layers 161 and 161 in the form of permanent magnets are formed for applying a bias magnetic field in a direction of arrow xcex1. Therefore, at portions of the ferromagnetic free layer 120 where the bias layers 161 and 161 abut the upper surfaces of the ferromagnetic free layer 120, the magnetization of the ferromagnetic free layer 120 is pinned in the direction of arrow a due to an exchange-coupling magnetic field. In FIG. 6, numerals 171 and 175 denote a pair of upper and lower electrodes, and numerals 181 and 185 denote a pair of upper and lower shield layers.
By adopting the head structure shown in FIG. 6, the problems generated in U.S. Pat. No. 5,729,410 could be solved. However, it was confirmed by the present inventors that new problems were generated in the head structure shown in FIG. 6.
Now, the ferromagnetic magneto-resistive tunnel effect (spin tunneling magneto-resistive effect) will be briefly explained. As the sense current is flowing perpendicularly to the surfaces of the TMR multilayered film 200, the conduction electrons are spin-polarized when they experienced the first ferromagnetic layer (20 or 40 depending on the current flowing direction). The probability of tunneling through the tunnel barrier layer is thus spin-dependent and depends upon the relative orientation of the two ferromagnetic layers 20 and 40 sandwiching the tunnel barrier layer. As illustrated in FIG. 5A, when the ferromagnetic layers 20 and 40 are parallel in magnetization to each other (or the relative magnetization angle therebetween is small), the density of states of majoritary spins is high in both layers, resulting in a high probability of electron tunneling through the tunnel barrier layer and a low junction resistance Rp. In constrast with this, as illustrated in FIG. 5C, when the ferromagnetic layers 20 and 40 are antiparallel in magnetization to each other (or the relative angle of magnetization therebetween is large), the density of states of majoritary spins is very different in each ferromagnetic layer, resulting in a low probability of electron tunneling through the tunnel barrier layer and a high junction resistance Rap. In the intermediate state between the state shown in FIG. 5A and the state shown in FIG. 5C, i.e. when both ferromagnetic layers are orthogonal in magnetization to each other, a resistance value Rm takes a value between the resistance value Rp and the resistance value Rap so that a relation of Rp less than Rm less than Rap is satisfied.
It was found through experiments implemented by the present inventors that an unfavorable phenomenon as shown in FIGS. 7A and 7B was generated between the ferromagnetic pinned layer and the ferromagnetic free layer in the head structure shown in FIG. 6. Specifically, as shown in FIG. 7A, when the magnetization directions of the ferromagnetic pinned layer 140 and the free layer 120 are basically parallel to each other, magnetization in both end portions 120a and 120a of the free layer 120 corresponding to regions xe2x80x9caxe2x80x9d in FIGS. 6 and 7A is fixed in the direction of arrow xcex1 due to the exchange-coupling relative to the bias layers as described above. If a sense current i is caused to flow in the laminate direction in this state, the current mainly flows at the center portions of the layers corresponding to region xe2x80x9cbxe2x80x9d in FIGS. 6 and 7A where the magnetization directions are parallel to each other and thus the resistance is small. The total resistance value at this time is given by Rxe2x80x2p. On the other hand, as shown in FIG. 7B, when the magnetization directions of the ferromagnetic pinned layer 140 and the free layer 120 are basically antiparallel to each other (also in this case, the magnetization in the end portions 120a and 120a of the free layer 120 is fixed in the direction of arrow xcex1 due to the exchange-coupling relative to the bias layers as described above), if a sense current i is caused to flow in the laminate direction, the current does not mainly flow at the antiparallel center portions of the layers, but branches to mainly flow at both end portions where the resistance is small (currents is and is). The total resistance value in FIG. 7B is given by Rxe2x80x2ap.
The resistance change ratio ((Rxe2x80x2apxe2x88x92Rxe2x80x2p)/Rxe2x80x2p) upon transition from the state of FIG. 7B to the state of FIG. 7A is smaller than the resistance change ratio ((Rapxe2x88x92Rp)/Rp) upon transition from the state of FIG. 5C to the state of FIG. 5A. As a result, there is raised a serious problem that the TMR (change) ratio is considerably lowered.
The present invention has been made under these circumstances and has an object to provide a magneto-resistive tunnel junction (TMR) head which can prevent the foregoing phenomenon wherein the current does not mainly flow at the antiparallel center portions of the layers but branches to mainly flow at both end portions where the resistance is low (the present inventors call this phenomenon xe2x80x9cextra current channel effectxe2x80x9d or xe2x80x9cthree current channel effectxe2x80x9d), so as to achieve a high head output for adaptation to ultrahigh density recording with less reduction in TMR ratio.
Another object of the present invention is to provide a magneto-resistive tunnel junction head which is excellent in flexibility of selection of biasing means.
For solving the foregoing problems, according to one aspect of the present invention, there is provided a magneto-resistive tunnel junction head having a tunnel multilayered film composed of a tunnel barrier layer, and a ferromagnetic free layer and a ferromagnetic pinned layer formed to sandwich the tunnel barrier layer therebetween, wherein the ferromagnetic free layer is applied with a bias magnetic field in a longitudinal direction thereof by biasing means disposed at and connected to longitudinal opposite ends thereof, and wherein a length of the ferromagnetic free layer in the longitudinal direction (bias magnetic field applying direction) thereof was set to be greater than a longitudinal length of the ferromagnetic pinned layer such that the ferromagnetic free layer is provided at the longitudinal opposite ends thereof with extended portions extending further beyond longitudinal opposite ends of the ferromagnetic pinned layer.
It is preferable that the biasing means located at the longitudinal opposite ends of the ferromagnetic free layer are contacted with upper or lower portions of the extended portions located at the longitudinal opposite ends of the ferromagnetic free layer, and that each of the biasing means is located with a predetermined space (D) from corresponding one of the longitudinal opposite ends of the ferromagnetic pinned layer.
It is preferable that the space (D) is set to a length which does not substantially lower a TMR ratio characteristic.
It is preferable that the space (D) is set to no less than 0.02 xcexcm.
It is preferable that the space (D) is set to no less than 0.02 xcexcm and no greater than 0.3 xcexcm.
It is preferable that the space (D) is set to no less than 0.02 xcexcm and less than 0.15 xcexcm.
It is preferable that the ferromagnetic free layer has a thickness of 20 xc3x85 to 500 xc3x85.
It is preferable that the tunnel multilayered film has a multilayered film detection end surface constituting an air bearing surface (ABS).
It is preferable that the ferromagnetic free layer is a synthetic ferrimagnet.
It is preferable that each of the biasing means is made of a highly coercive material or an antiferromagnetic material, or in the form of a laminate body having an antiferromagnetic layer and at least one ferromagnetic layer.
It is preferable that a pinning layer for pinning magnetization of the ferromagnetic pinned layer is stacked on a surface of the ferromagnetic pinned layer remote from a side thereof abutting the tunnel barrier layer.
It is preferable that the tunnel multilayered film is electrically contacted with a pair of electrodes which are disposed to sandwich the tunnel multilayered film therebetween.
It is preferable that a pair of shield layers are formed to sandwich the pair of electrodes therebetween.
It is preferable that longitudinal opposite ends of the tunnel multilayered film are insulated by insulating layers.